1. Field of the Invention
This invention relates, generally, to methods for casting ophthalmic lenses. More particularly, it relates to a method that reduces the number of gaskets that a lens maker must maintain in inventory.
2. Description of the Prior Art
The optical characteristics of an ophthalmic lens can be varied by the curvature of its two surfaces, the thickness at its center and edges, and its diameter. The two surfaces of a lens can use several different geometric configurations. One example is a toric surface that resembles the lateral surface of a torus and which has a barrel shape. A toric surface is similar to a cylindrical surface, but the longitudinal axis curves instead of being straight as in the case of a cylindrical surface. The perpendicular axis or meridian on the toric surface has a radius of curvature smaller that the radius of the axis. As with a cylindrical surface, a toric surface can be convex by having the shape of the exterior surface of a torus or, alternatively, may be concave by having the shape of the inner surface of a torus. In addition to the toric and cylindrical geometric shapes, lenses may also be spherical, piano, elliptical, or progressive.
The desired optical characteristic of a lens may include a combination of the different geometric shapes. For example, a lens may have a spherical front surface and a toric back surface. An eyeglass lens used to correct astigmatism is a toric lens. It is curved in such a way as to have a different focal length along each axis. A common lens configuration includes a convex spherical front surface and a concave toric back surface.
Plastic ophthalmic lenses are typically fabricated by casting and curing a transparent plastic material within a casting cell. The casting cell includes a front mold and a back mold that fit together within a cylindrical gasket. Each mold has a polished inner surface with desired optical characteristics that are transferred, respectively, to either the front or rear of a plastic lens during the casting process. The cylindrical gasket provides a seal for the molds and forms the edge of the lens. The casting cell is filled with plastic and cured using heat or ultraviolet radiation.
The optical characteristics that each mold provides to a lens vary significantly. Therefore, one mold cannot be used to account for every possible variation that may be required to meet corrective ophthalmic specifications. Therefore, a lens is produced that has a concave, unfinished side that is surfaced after the curing process is completed. Thus, instead of forming the lens to be mounted into glasses with a few finishing actions, the semi-finished lens has only a single finished surface formed by a mold and the other surface is mechanically finished after the lens has been cured. The semi-finished lenses, accordingly, are made in stages, in which one surface is finished by a mold and the other surface is machine finished after curing. The surface of the lens formed by a mold is usually the front spherical surface, with or without add power.
Attempts have been made at eliminating the labor involved with transforming a semi-finished lens to a finished lens by creating a lens that is formed with both surfaces finished. Each different optical characteristic required for a particular corrective lens necessitates several combinations of a front mold, back mold, and gasket. A lens that is cast in finished form represents significant savings in the cost of manufacturing a lens with the reduction of the labor involved in producing a finished lens. However, producing finished lenses through casting requires a voluminous number of molds and gaskets to produce the range of different corrective lenses and is therefore impractical.
For example, the prior art gasket that receives the toric back mold must have a toric seat that is a complement to that particular toric back mold. Accordingly, a large inventory of toric molds and corresponding toric gaskets must be maintained to enable the making of the vast range of corrective toric lenses.
Continuing efforts are being made to improve the casting of plastic lenses to produce finished lenses. By way of example, U.S. Pat. No. 4,522,768 to Roscrow et al. discloses an assembly that allows adjustment of a first mold so that a multiplicity of desired optical characteristics can be made using a limited number of casting dies and gasket members. However, more than one gasket is required to achieve all desired optical characteristics. U.S. Pat. No. 5,160,749 to Fogarty discloses a leak resistant seal for a mold assembly for making a lens. Fogarty does not provide for producing toric lenses, but rather only a lens blank that must be ground and polished to the desired optical characteristics. U.S. Pat. No. 6,103,148 to Su et al. discloses a gasket that can accommodate a plurality of molds but does not teach a method that incorporates an improved and simplified sealing mechanism for the gasket.
There is a need, therefore, for a method that simplifies the making of a toric lens. More particularly, there is a need for a method that would reduce the number of toric gaskets that would need to be maintained in inventory. Moreover, there is a need for a method that facilitates sealing of the toric lens mold as well.
However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the pertinent art how the identified needs could be fulfilled.
The longstanding but heretofore unfulfilled need for an improved method of casting toric lenses is now met. The new, useful, and nonobvious method includes the steps of providing a casting cell that includes a front mold, a toric back mold, and a gasket. An annular carrier ring is formed integrally with the toric back mold, and a radially-inwardly extending annular wall is formed in the gasket. A flat first step is formed where a front edge of the annular wall meets the gasket at a right angle and a flat second step is formed where a rear edge of the annular wall meets the gasket at a right angle.
In a first embodiment, the annular carrier ring is formed on the rear or lower end of the toric back mold and a right angle is formed where the peripheral edge of the toric back mold and the annular carrier ring meet.
The first flat step is adapted to squarely abut and support a peripheral edge of the front mold when the casting cell is assembled.
The second flat step is adapted to squarely abut and support the annular carrier ring of the toric back mold when the casting cell is assembled.
When the casting cell is assembled, the peripheral edge of the front mold is positioned in squarely abutting relation to the first flat step and the annular carrier ring of the toric back mold is positioned in squarely abutting relation to the second flat step. A cavity defined by a back surface of the front mold and a front surface of the toric back mold is filled with a reactive resin and the resin is cured.
In a second embodiment, the annular carrier ring and the toric back mold are integrally formed with one another and the annular carrier ring is coextensive with a peripheral edge of the toric back mold. A square, annular step is formed in the peripheral edge of the toric back mold. A radially-inwardly extending annular wall is formed in the gasket so that a first flat step is formed where a front edge of the annular wall meets the gasket at a right angle and so that a second flat step is formed where a rear edge of the annular wall meets the gasket at a right angle. The peripheral edge of the front mold is positioned in squarely abutting relation to the first flat step and the square, annular step formed in said peripheral edge of the toric back mold is positioned in squarely abutting relation to the second flat step.
As in the first embodiment, a cavity defined by a back surface of the front mold and a front surface of the toric back mold is filled with a reactive resin and the resin is cured.
In a third embodiment, the annular carrier ring is formed integrally with a peripheral edge of the toric back mold and may have any predetermined geometrical configuration. An annular recess having a matching geometric configuration is formed in the gasket so that the toric back mold and gasket are snap-fit together. In this embodiment, the radially-inwardly extending annular wall of the first embodiment provides the first flat step on the front edge of the annular wall and the front mold is fitted thereto as in the first embodiment. The second flat step of the first embodiment is eliminated.
A primary object of the invention is to provide an improved method of casting toric lenses.
Another very important object is to provide a method that reduces the number of toric gaskets that must be maintained in inventory by a lens maker.
Still another important object is to provide a method that produces toric lenses having a higher quality that conventionally cast toric lenses.
These and other important objects, advantages, and features of the invention will become clear as this description proceeds.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts that will be exemplified in the description set forth hereinafter and the scope of the invention will be indicated in the claims.